This invention relates to a land vehicle suspension control system.
By "land vehicle" is meant a vehicle adapted for motion over the land in contact therewith, examples being motor cars, motor cycles, tractors and tracked vehicles.
In particular, the invention relates to a land vehicle suspension control system for a land vehicle having an active suspension system.
An active suspension system is a suspension system in which conventional suspension components, such as springs and dampers, are assisted or replaced by actuators operable, for example in response to command signals from a microprocessor, to correct, change or control the attitude of the vehicle. An aim of such active suspension systems is to minimize variations in the forces experienced by the vehicle body, thereby improving vehicle safety and enhancing driver and passenger comfort.
The command signals which control the actuators are generally derived from measured values of a number of variables defining the vehicle's attitude. In a truly active suspension system, there is the capability not merely of controlling the actuators in response to previously measured changes in the values of the variables, but also of, for example, inherently biasing the attitude of the vehicle to offset the effects of a known steady-state or dynamic loading, or of controlling the actuators in a manner predictive of expected land conditions.
Active suspension systems are now well known. For example, EP-A-0114757 discloses an active suspension system for a four-wheeled motor vehicle in which force measurements are taken at the points of support of the vehicle body on each wheel/hub assembly and processed to produce a demanded output of the actuator secured to operate between the respective wheel/hub assembly and the vehicle body.
The attitude of the vehicle can then- be controlled by converting the forces measured at the points of support to a set of modal forces (e.g. heave, pitch, roll and warp forces), from which the actuator outputs required to overcome the combined modal forces in order to maintain the desired attitude of the vehicle are then calculated.
A significant advantage of such an active suspension system is that the suspension characteristics of the vehicle can be continuously altered to accommodate varying road conditions and/or operating conditions of the vehicle. This facility permits the construction of a vehicle which has improved safety characteristics, since it is possible to maintain a greater degree of contact of the vehicle wheels with the land, and the behaviour of the vehicle is likely to be more predictable to the driver, than in the case of a vehicle not having an active suspension system.
A disadvantage of known active suspension systems is that the actuators employed between the vehicle body and the wheel/hub assemblies to apply force under the command of the microprocessor are imperfect in that they are not capable of implementing the command signals of the microprocessor accurately. For example, there is always a finite time delay between an actuator receiving a co=and signal and the piston of the actuator moving in response to the command signal.
Since the control system of an active suspension system operates very rapidly, such time delays cannot be tolerated since, for example, the resulting delayed operation of an actuator in response to a first command of the microprocessor may interfere with the implementation of a subsequent command.
The output of an actuator in an active suspension system can be smoothed by the inclusion of an isolator block of, for example, rubber between the actuator and the body of the vehicle. Such isolator permits transmission only of loads of greater than a certain magnitude.
However, a disadvantageous effect of the inclusion of such isolators is that the vehicle body may move without the active suspension system either detecting or responding to the load causing such movement. This is because the isolators isolate the loads from the load cells of the system, which are usually disposed to sense loads acting between the actuator and the isolator. Thus, the isolators act as a separate passive suspension system within the active suspension system, and introduce an extra degree of freedom into the load path between the vehicle body and the road surface on which the vehicle stands or travels.
It has hitherto been impossible to accurately control the extension of an actuator having an isolator disposed between it and the vehicle body, since accurate feedback measurement of the actuator response to a commanded signal is not possible when the isolator at least partially isolates the loads which it is required to measure from the load cell of the actuator.